ConstraintKinds, Functor, Set

ConstraintFunctor.hs

-- | https://jeltsch.wordpress.com/2013/02/14/the-constraint-kind/
{-# LANGUAGE ConstraintKinds, TypeFamilies #-}

import           Prelude hiding (Functor(..))
import           Data.Set    (Set)
import qualified Data.Set as Set
import           GHC.Exts    (Constraint)

class Functor f where
  type Object f a :: Constraint
  type Object f a = ()
  fmap :: (Object f a, Object f b) => (a -> b) -> f a -> f b

instance Functor Set where
  type Object Set a = Ord a
  fmap = Set.map